Multiple voltage level driving for electrophoretic displays

ABSTRACT

This application is directed to driving methods for electrophoretic displays. The driving methods comprise applying different voltages selected from multiple voltage levels, to pixel electrodes and optionally also to the common electrodes. In a preferred method, the different voltages are selected from a group consisting of 0V, at least two levels of positive voltage and at least two levels of negative voltage.

The present application claims the benefit of U.S. ProvisionalApplication 61/148,746, filed Jan. 30, 2009, which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to methods comprising applying a voltageselected from multiple voltage levels to drive an electrophoreticdisplay.

BACKGROUND OF THE INVENTION

An electrophoretic display (EPD) is a non-emissive device based on theelectrophoresis phenomenon of charged pigment particles suspended in asolvent. The display usually comprises two plates with electrodes placedopposing each other. One of the electrodes is usually transparent. Asuspension composed of a colored solvent and charged pigment particlesis enclosed between the two plates. When a voltage difference is imposedbetween the two electrodes, the pigment particles migrate to one side orthe other, according to the polarity of the voltage difference. As aresult, either the color of the pigment particles or the color of thesolvent may be seen at the viewing side. An EPD may be driven by auni-polar or bi-polar approach.

However, the driving methods currently available pose a restriction onthe number of grayscale outputs. This is due to the fact that displaydriver ICs and display controllers are limited in speed on the minimumpulse length that a waveform can have. While current active matrixdisplay architectures utilize ICs that can generate pulse lengths downto 8 msec leading to electrophoretic displays which have shortenedresponse time, even below 150 msec, the grayscale resolution seems todiminish due to the incapability of the system to generate shorter pulselengths.

SUMMARY OF THE DISCLOSURE

The present invention is directed to methods for driving anelectrophoretic display, which method comprises applying differentvoltages selected from multiple voltage levels, to pixel electrodes andoptionally also to the common electrode.

The method allows for multiple voltage levels, specifically, 0 volt, atleast two levels of positive voltage and at least two levels of negativevoltage.

The method can provide finer control over the driving waveforms andproduce a better grayscale resolution.

The first aspect of the invention is directed to a driving method for adisplay device comprising an array of display cells wherein each of saiddisplay cells is sandwiched between a common electrode and a pixelelectrode, which method comprises applying different voltages selectedfrom a group consisting of 0 V, at least two levels of positive voltageand at least two levels of negative voltage, to the pixel electrode. Inone embodiment, the different voltages are selected from a groupconsisting of 0V, three levels of positive voltage and three levels ofnegative voltage. In one embodiment, the different voltages are selectedfrom a group consisting of 0V, −5V, −10V, −15V, +5V, +10V and +15V. Inone embodiment, the voltage applied to the common electrode remainsconstant. In another embodiment, the method further comprises applyingdifferent voltages selected from a group consisting of 0V, at least twolevels of positive voltage and at least two levels of negative voltage,to the common electrode. The different voltages applied to the commonelectrode are selected from a group consisting of 0V, three levels ofpositive voltage and three levels of negative voltage. In oneembodiment, the different voltages applied to the common electrode areselected from a group consisting of 0V, −5V, −10V, −15V, +5V, +10V and+15V. In one embodiment, the display device is an electrophoreticdisplay device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a typical electrophoretic displaydevice.

FIG. 2 illustrates an example of a driving method of the presentinvention.

FIG. 3 illustrates an example of an alternative driving method of thepresent invention.

FIG. 4 is a table which shows the possible voltage combinations in amethod of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a typical array of electrophoretic display cells 10a, 10 b and 10 c in a multi-pixel display 100 which may be driven by anyof the driving methods presented herein. In FIG. 1, the electrophoreticdisplay cells 10 a, 10 b, 10 c, on the front viewing side, are providedwith a common electrode 11 (which is usually transparent). On theopposing side (i.e., the rear side) of the electrophoretic display cells10 a, 10 b and 10 c, a substrate (12) includes discrete pixel electrodes12 a, 12 b and 12 c, respectively. Each of the pixel electrodes 12 a, 12b and 12 c defines an individual pixel of the multi-pixelelectrophoretic display 100, in FIG. 1. However, in practice, aplurality of display cells (as a pixel) may be associated with onediscrete pixel electrode. The pixel electrodes 12 a, 12 b, 12 c may besegmented in nature rather than pixellated, defining regions of an imageto be displayed rather than individual pixels. Therefore, while the term“pixel” or “pixels” is frequently used in this disclosure to illustratedriving implementations, the driving implementations are also applicableto segmented displays.

An electrophoretic fluid 13 is filled in each of the electrophoreticdisplay cells 10 a, 10 b, 10 c. Each of the electrophoretic displaycells 10 a, 10 b, 10 c is surrounded by display cell walls 14.

The movement of the charged particles in a display cell is determined bythe voltage potential difference applied to the common electrode and thepixel electrode associated with the display cell.

As an example, the charged particles 15 may be positively charged sothat they will be drawn to a pixel electrode (12 a, 12 b or 12 c) or thecommon electrode 11, whichever is at an opposite voltage potential fromthat of charged particles 15. If the same polarity is applied to thepixel electrode and the common electrode in a display cell, thepositively charged pigment particles will then be drawn to the electrodewhich has a lower voltage potential.

In another embodiment, the charged pigment particles 15 may benegatively charged.

The charged particles 15 may be white. Also, as would be apparent to aperson having ordinary skill in the art, the charged particles may bedark in color and are dispersed in an electrophoretic fluid 13 that islight in color to provide sufficient contrast to be visuallydiscernable.

The electrophoretic display 100 could also be made with a transparent orlightly colored electrophoretic fluid 13 and charged particles 15 havingtwo different colors carrying opposite particle charges, and/or havingdiffering electro-kinetic properties.

The electrophoretic display cells 10 a, 10 b, 10 c may be of aconventional walled or partition type, a microencapsulted type or amicrocup type. In the microcup type, the electrophoretic display cells10 a, 10 b, 10 c may be sealed with a top sealing layer. There may alsobe an adhesive layer between the electrophoretic display cells 10 a, 10b, 10 c and the common electrode 11.

FIG. 2 shows a driving method of the present invention. In this example,the voltage applied to the common electrode remains constant at the 0volt. The voltages applied to the pixel electrode, however, fluctuatesbetween −15V, −10V, −5V, 0V, +5V, +10V and +15V. As a result, thecharged particles associated with the pixel electrode would sense avoltage potential of −15V, −10V, −5V, 0V, +5V, +10V or +15V.

FIG. 3 shows an alternative driving method of the present invention. Inthis example, the voltage on the common electrode is also modulated. Asa result, the charged particles associated with the pixel electrodeswill sense even more levels of potential difference, −30V, −25V, −20V,−15V, −10V, −5V, 0V, +5V, +10V, +15V, +20V, +25V and +30V (see FIG. 4).While more levels of potential difference are sensed by the chargedparticles, more levels of grayscale may be achieved, thus a finerresolution of the images displayed.

The common electrode and the pixel electrodes are separately connectedto two individual circuits and the two circuits in turn are connected toa display controller. In practice, the display controller issues signalsto the circuits to apply appropriate voltages to the common and pixelelectrodes respectively. More specifically, the display controller,based on the images to be displayed, selects appropriate waveforms andthen issues signals, frame by frame, to the circuits to execute thewaveforms by applying appropriate voltages to the common and pixelelectrodes. The term “frame” represents timing resolution of a waveform.

Although the foregoing disclosure has been described in some detail forpurposes of clarity of understanding, it will be apparent to a personhaving ordinary skill in that art that certain changes and modificationsmay be practiced within the scope of the appended claims. It should benoted that there are many alternative ways of implementing both theprocess and apparatus of the improved driving scheme for anelectrophoretic display, and for many other types of displays including,but not limited to, liquid crystal, rotating ball, dielectrophoretic andelectrowetting types of displays. Accordingly, the present embodimentsare to be considered as exemplary and not restrictive, and the inventivefeatures are not to be limited to the details given herein, but may bemodified within the scope and equivalents of the appended claims.

1. A driving method for a display device comprising an array of displaycells wherein each of said display cells is sandwiched between a commonelectrode and a pixel electrode, which method comprises applyingdifferent voltages selected from a group consisting of 0V, at least twolevels of positive voltage and at least two levels of negative voltage,to the pixel electrode.
 2. The driving method of claim 1 wherein saiddifferent voltages are selected from a group consisting of 0V, threelevels of positive voltage and three levels of negative voltage.
 3. Thedriving method of claim 2 wherein said different voltages are selectedfrom a group consisting of 0V, −5V, −10V, −15V, +5V, +10V and +15V. 4.The driving method of claim 1 wherein the voltage applied to the commonelectrode remains constant.
 5. The driving method of claim 1 furthercomprising applying different voltages selected from a group consistingof 0V, at least two levels of positive voltage and at least two levelsof negative voltage, to the common electrode.
 6. The driving method ofclaim 5 wherein said different voltages are selected from a groupconsisting of 0V, three levels of positive voltage and three levels ofnegative voltage.
 7. The driving method of claim 6 wherein saiddifferent voltages are selected from a group consisting of 0V, −5V,−10V, −15V, +5V, +10V and +15V.
 8. The driving method of claim 1 whereinsaid display device is an electrophoretic display device.